For optimum performance and efficiency of a marine controllable pitch propeller ("CPP") it is necessary to control the pitch as accurately as possible. Most control systems for CPP's rely on the physical fore-and-aft position of a part of a translating member of the pitch control mechanism that is within the vessel hull to obtain an indication of the propeller pitch.
There are various designs of CPP's in common use. For example, one design employed by manufacturers of CPP's for many years consists of a hub and blade assembly in which the pitch is controlled by a hydraulic piston and crosshead assembly in the hub. The piston drives a crosshead in a fore-and aft direction which, in turn, pivots the blades by rotating the blade mounts through an arc by means of an eccentric pin/sliding block/slot arrangement or, in a variation of this design, by means of a system of mechanical links connecting the crosshead and blade mounts. The piston movement is controlled by a directional actuating valve, the valve body of which is affixed to the piston-crosshead assembly and the spool of which is affixed to the aft end of an oil transmitting tube (also commonly called the "valve rod"). The valve rod extends forwardly within the propeller shaft into the hull of the vessel and to a device usually referred to as the "oil distribution box" or "0.D. box." The 0.D. box includes rotary seals and oil passages for transmitting the hydraulic actuating oil to and from the hydraulic piston in the hub through the rotating propeller shaft and the rotating/translating valve rod.
The 0.D. box also includes an actuating device, usually a hydraulic servomotor, that moves the valve rod, and hence the valve spool, fore and aft in response to control signals from the remote control apparatus of the CPP system. The directional actuating valve directs hydraulic actuating oil to the selected side of the piston, and the piston-crosshead assembly translates in the desired direction to effect a pitch change of the blades. When the valve rod has been moved by the actuating device in the 0.D. box to a position such as to establish the desired propeller pitch, the movement of the valve rod is stopped. The piston-crosshead assembly continues moving a small distance until the valve spool is centered in the valve body, at which point the flow of oil to the piston and movement of the piston/crosshead assembly are stopped.
The valve rod translates with the piston and crosshead assembly. The fore-and-aft position of the inboard end of the valve rod provides, therefore, an indication of the position of the piston-crosshead assembly in the hub and thereby of the pitch of the blades. Pitch indication is provided in most systems by means of a pointer attached to the forward end of the valve rod and a pitch scale attached to the oil distribution box. In addition most systems have a device associated with the forward end of the valve rod for generating a pneumatic or electrical signal used for remote indication of the pitch-setting and as a feedback signal for processing in the control system to maintain a desired pitch.
The oil distribution box can be located either in the shaft line or forward of the reduction gear box at the forward end of the shaftline. The pitch-indicating scale and the stationary parts of the remote-signalling device are usually affixed to the oil distribution box and, in effect, also affixed to the hull (via the shaftline and thrust bearing for 0.D. boxes located in the shaftline and via the reduction gear casing for 0.D. boxes located on the reduction gear casing). The thrust bearing is in the shaftline and is, therefore, located aft of the 0.D. box for 0.D. boxes mounted on the reduction gear box at the forward end of the shaftline and can be located either forward or aft of the 0.D. box for shaft-mounted 0.D. boxes. The pointer of the pitch-indicating scale is affixed to the valve rod, and the movable parts of the remote-signalling device are coupled to the valve rod. The propeller is, of course, located outboard at the aft end of the propeller shaft. In some ships the distance between the pitch-indicating scale and remote-signalling device and the propeller is more than 300 feet.
Inaccuracies in the pitch-indication system result from the fact that both the valve rod and the propeller shaft are subject to changes in length due to changes in propeller load, hydraulic oil pressure, and oil, sea, and air temperatures. With installations where the 0.D. box is located on the reduction gear casing, additional inaccuracies result from the displacement of the propeller/valve rod/hub assembly with respect to the pitch scale and remote-signalling device due to thrust bearing manufacturing clearances and normally permitted thrust bearing wear-down limits. Inaccuracies due to changes in shaft and valve rod length caused by steady-state loads and thrust bearing wear-down are predicable and can, to some extent, be compensated for in the designs of the scale and the remote-signalling and processing system. However, inaccuracies due to temperature changes and to transient load changes, such as those associated with vessel speed changes in magnitude and/or direction, cannot be readily designed into the scale or readily compensated for in the control system logic.
Changes in temperature are not easily dealt with. One can imagine sensing the water temperature as indicative of the actual shaft and valve rod temperatures and repositioning the scale (or pointer) to adjust for changes in length due to such temperature changes. Such an approach, however, is complicated and only partly solves the problem, because the lengths of the shaft and the valve rod are affected by temperature conditions other than the water temperature, such as the temperature of the oil supplied to the hub for positioning the piston/crosshead as well as the ambient air temperature around the part of the shaft within the hull.
Inaccuracies in indicated and detected pitch similar to those described above are present in other types of CPP systems. In CPP systems in which the actuating valve for the hydraulic piston in the hub is within the vessel (either inside or outside the shaft), the translatable oil supply tube and propeller shaft, with which the pitch-indicating and pitch-detecting devices are associated, are subject to the variations in length described above. So also are the pitch-indicating and pitch-detecting devices of force-rod type CPP systems subject to inaccuracies resulting from changes in the lengths of the shaft and force rod due to load and temperature variations that are not easily compensated for.
In summary the conditions that affect the inaccuracy of inboard pitch indication by detection of the position of an inboard part of a valve rod, oil supply tube or force rod relative to a scale affixed, in effect, to the hull vary considerably, and monitoring such conditions and compensating for them in the pitch detection and control system would introduce complications, increase costs and be only partly successful in eliminating inaccuracies. The actual pitch-setting of the propeller is represented accurately by the fore-and-aft position of the aft end of the valve rod relative to the propeller hub (except for a small and normally acceptable inaccuracy due to variations in valve spool centering location). The inaccuracies in propeller pitch indicated by the scale and signalled by the remote-signalling device result from the variations described above in the lengths of the shaft and the valve rod between the hub and the components of the scale and the remote-signalling device.
Consideration has often been given to providing a device within the propeller hub to detect the position of the valve rod (or oil tube or force rod), crosshead or other translating member of the pitch control mechanism. Prior suggestions for such a device have been rejected for various reasons. One important reason is the hostile and inaccessible environment of the detecting device. The hub usually contains oil or grease, which rules out optical detectors and can cause problems for electrical detector devices. The hub and blade assembly is subject to vibration and shock that can cause failure of the device. If the device fails, replacement is difficult at best and would almost certainly have to be done in port. If the detecting device has contacting, relatively moving parts (e.g., a potentiometer), wear inevitably will produce failure and necessitate replacement. Another reason is the complication of transmitting reliably information indicative of the detected position of the translating member from a detecting device in the hub into the vessel.